Intra-cavity laser absorption spectroscopy (ICLAS) is a technique used to detect very low concentrations of certain atomic and molecular species. A laser generally comprises a gain medium, a resonator cavity, and an energy source, called a pumping source. A conventional resonator cavity is constructed with two mirrors that reflect light back and forth between one another, with the gain medium in between. The gain medium is stimulated by light from the pumping source, and the light is amplified as it passes through the gain medium. The amplified light is then reflected by one of the mirrors, whereupon it passes through the gain medium again and is further amplified. As the light is reflected back and forth between the two mirrors, the light is continually amplified until reaching some steady state, ultimately resulting in a coherent beam of light or other electromagnetic (EM) radiation.
In ICLAS, a sample of interest is placed inside the resonator cavity of a laser. The mirrors confine most of the optical radiation emitted by the gain medium to the cavity of the laser, creating a multi-pass absorption effect in the sample as light is reflected and travels back and forth between the mirrors and through the sample. As the light travels through the sample, some wavelengths of the light may be absorbed by the sample more than others. When the light then returns to pass through the gain medium, the attenuation of the light at certain wavelengths as a result of the absorption in the sample causes the light at those wavelengths to be amplified less than light at wavelengths not absorbed by the sample. As the light is repeatedly fed back through the sample and the gain medium for amplification, incremental attenuation of light at wavelengths absorbed by the sample results in a larger total attenuation that can be observed with optical sensors. Because ICLAS requires that the sample be placed within the resonator cavity of the laser, ICLAS is an in-situ measurement technique primarily suitable for measurement of samples in laboratory environments.